Field of the Invention
The present invention relates to a CMOS image sensor, and more particularly to a semiconductor photosensitive unit and a semiconductor photosensitive unit array thereof.
Description of Related Art
The existing image sensors are mainly divided into two types: charge-coupled device image sensors and CMOS image sensors. The charge-coupled device image sensors have advantages such as high image quality and low noise, but have high production cost and are difficult to be integrated with a peripheral circuit. The CMOS image sensors have high integration level, small volume, low power consumption, and a wide dynamic range, are compatible with the manufacturing process of the charge-coupled device image sensors, and meet the condition of high system integration. Therefore, the CMOS image sensors have become a research focus in recent years.
FIG. 1 illustrates an existing CMOS image sensor composed of a circuit of a single pixel unit. A single pixel unit of the CMOS image sensor has four MOS transistors and specifically includes: a photodiode (PD), a charge overflow gate transistor (TG), a reset transistor (RST), a source follower (SF), and a selector transistor (RS); its working process is as follows: firstly, entering a “reset state”, in which the reset transistor is switched on to reset the photodiode; then, entering a “sampling state”, in which the reset transistor is switched off, and photon-generated carriers are produced when light is irradiated onto the photodiode and are amplified and output by means of the source follower; and finally, entering a “read state”, in which the selector transistor is switched on and signals are output via a column bus. The defect of the CMOS image sensor is that, the four independently working MOS transistors occupy a large substrate area in the single pixel unit of the CMOS image sensor, the product pixel is low, and the product resolution is not high.
To overcome the defect of the existing CMOS image sensor, Chinese Patent No. 200910234800.9 discloses a “Planar-Channel Semiconductor Photosensitive Device”, and its cross-sectional diagram along the length direction of a current channel is illustrated in FIG. 2. The function of a semiconductor photosensitive device 10 is enabled by charging or discharging a floating gate using a photosensitive pn junction diode disposed between a floating gate region 505 and a drain 514, thereby simplifying the structure of the semiconductor photosensitive device and also improving the resolution of the image sensor. However, to guarantee the performance of the semiconductor photosensitive device, the planar-channel semiconductor photosensitive device also requires a long current channel, which increases the area of the semiconductor photosensitive device to some extent and reduces the chip density. To overcome this defect, Chinese Patent Application No. 201310513086.3 discloses a U-shaped channel semiconductor photosensitive device, wherein on the basis that a photosensitive pn junction diode is used to charge or discharge a floating gate, a current channel region is recessed in a semiconductor substrate, which can reduce the size of the device while extending the current channel region. However, the above two structures of semiconductor photosensitive devices that charge or discharge a floating gate by using a photosensitive pn junction diode have a common problem: a photosensitive region of the photosensitive pn junction diode requires a large area, and the floating gate is directly connected to the photosensitive region of the photosensitive pn junction diode, so that after being charged into the floating gate, the photosensitive current is easily leaked to the photosensitive region of the photosensitive pn junction diode, which directly affects the working reliability of the image sensor device.